This invention relates to an optical information-recording/reproducing apparatus with cancellation for an offset contained in an error signal, and more particularly, to an optical information-recording/reproducing apparatus suitable for, for instance, an optical disc device or an optical magnetic disc device which records, reproduces, erases, and rewrites information by irradiating a beam to the information-recording medium (hereinafter called a disc).
Generally, in an optical disc device or an optical magnetic disc device, each concentric or spiral track on a disk is divided into plural sectors per circle, to form the pre-format portion for use as the address index, and the recording, reproduction, erasing, and rewriting of information are made in the data portion, i.e., the portion other than the pre-format portion. For reasons of workability, etc., such a pre-format portion is usually provided by opening pits (a dented or hollow portion) in advance (pre-pits) in the disc, by means of a pressing or stamping method. However, in an error signal, i.e. a focus-error signal (FES) or a track-error signal (TES), obtained by detecting the return beams coming from the disc provided with such a pre-format portion as in the above method, there appears the phenomenon of the different DC offset components of the pre-format portion and the data portion overlapping each other. Such overlapping of the different DC offset parts is observed during the recording operation of a draw-type optical disc and during both the recording and reproduction operations of an optical magnetic disc.
FIGS. 1(A) and 1(B) show the above-mentioned relations in the prior art; FIG. 1(A) shows the construction of the closed servo-loop for tracking control of an optical disc device, and FIG. 1(B) illustrates the conditions of the DC-offset error signal in the servo-loop. In FIG. 1(A), numeral 1 denotes a disc which rotates by means of spindle motor 11. The laser beam from semiconductor laser 2 is irradiated to disc 1 by means of collimeter lens 21, half-mirror 22, and object lens 23. Numeral 4 shows a 2-or 4-split beam detector for detecting the return beams, whose output signal is input to differential amplifier 5, to obtain error signal 6. Error signal 6 is input to coil 9 in an actuator (not shown), via phase-compensator circuit 7 and driving-circuit 8 for driving the beam-moving means. This moves object lens 23 and thus controls the beams. Error signal 6 (obtained by means of detection by beam detector 4) of the return beam returned from optical disc 1, now has DC-offset components which differ between the pre-format portion and the data portion. FIG. 1(B) shows a diagram illustrating the conditions of the DC-offset components at the pre-format portion and the data portion, wherein numeral 61 shows the DC offset, 611 the offset at the pre-format portion, and 612 the offset at the data portion. In the above closed servo-loop, however, error signal 6 having the offset moves object lens 2 by means of coil 9 via driving circuit 8, and the loop makes the control so as to electrically compensate for error signal 6, which is the output from differential amplifier 5, to reduce it to zero. Then the error signal becomes as shown by numeral 62 in FIG. 1(B), and as a result, the actual beams will be subjected to tracking control at a point off from the ideal central position on the track, to the extent that the offset is cancelled. The same is true in the case of the focus-error signal, in which focussing control will be performed by the actual beam defocussed off the disc.
Regarding such a change of offset in the error signals, the applicant of this invention once proposed the idea of constantly controlling such change based on a write gate signal, during the time when information is recorded in an optical disc device, to ensure a stable and reliable writing operation.
According to the prior art shown in FIG. 1(A), however, the change of offset contained in the error signals is electrically compensated for, by the closed servo-loop, so as to control the actual beams at a defocussed or off-track position. Further, the above-mentioned method of making the offset constant, based on the write gate signal only during the recording time, is ineffective in the case of an optical magnetic disc which is largely affected by the offset of the error signals at reproduction time, too. As a result, the prior art has the problem that normal tracking and focussing are impossible at the recording or reproduction time, thus worsening the error rate when performing the information processing.